Field of the Invention
The invention relates to a charge pump.
Various charge pumps are described in U.S. Pat. Nos. 4,740,715, 5,126,590, 5,202,588 and 5,343,088. The basic operating principle of a charge pump is now described.
A charge pump has two pumping capacitors to each of which an input clock signal is fed at an electrode. Electrodes of the pumping capacitors remote from the input clock signals are connected to ground via transistors and are connected to an output of the charge pump via other transistors. The transistors are p-channel type transistors. The charge pump feeds a load, which has a load capacitance. The control terminals of the transistors are connected to different control signals.
With each clock pulse edge of the input clock signals there is a charge reversal of the electrodes of the pumping capacitors remote from the inputs. As this happens, one electrode must be pumped from the value of the output potential to a positive value and the other electrode must be pumped from 0V to a negative value.
It is accordingly an object of the invention to provide a charge pump that overcomes the above-mentioned disadvantages of the prior art devices of this general type, with which a more negative output potential can be produced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a charge pump. The charge pump contains two inputs, each for receiving an input clock signal; an output for outputting a pumped output potential; a first circuit module; a second circuit module connected to the output; a supply potential terminal connected to the first circuit module; and two pumping capacitors including a first pumping capacitor and a second pumping capacitor each having a first electrode and a second electrode. The first electrode of each of the pumping capacitors is connected to one of the inputs, and the second electrode of the pumping capacitors is connected to both the first circuit module and the second circuit module. The second electrode in each case is coupled through the first circuit module to the supply potential terminal and through the second circuit module to the output. A controllable short-circuiting element is provided and has a control terminal and a controllable path with a first end connected to the second electrode of the first pumping capacitor and a second end connected to the second electrode of the second pumping capacitor. A first switching element is provided and has a first terminal connected to the second electrode of the first pumping capacitor, a second terminal connected to the control terminal of the controllable short-circuiting element, and a control terminal. A second switching element has a first terminal connected to the second electrode of the second pumping capacitor, a second terminal connected to the control terminal of the controllable short-circuiting element, and a control terminal.
The charge pump according to the invention has a controllable short-circuiting element, the controllable path of which is disposed between the two electrodes of the two capacitors that are remote from the inputs of the charge pump. By the short-circuiting element it is possible in an advantageous way to carry out a charge equalization between the two electrodes of the two pumping capacitors at any desired points in time.
It is particularly favorable if the short-circuiting element is driven by its control terminal in such a way that, immediately before clock pulse edges of the input clock signals, it is conducting, and brings about the charge equalization, and is subsequently blocked again before the clock pulse edges occur. During every pumping period, the potentials of the second electrodes of the pumping capacitors are initially equalized via the short-circuiting element to their arithmetic mean value, before their level is changed by the pumping of the input clock signals. This results in an increase in the absolute amount of the peak values of the potentials at the second electrodes. For this reason, an output potential of a greater absolute amount than without the short-circuiting according to the invention is achieved. The short-circuiting has the effect that a change in potential already takes place to a certain extent at the second electrodes in the direction of the subsequent increase or decrease in potential induced by the input clock signals. As a result, potentials of a greater absolute value are subsequently produced at the second electrodes by the excursion of the input clock signals.
According to a development, the first and second circuit modules are non-conducting whenever the short-circuiting element is conducting. This prevents the supply potential and the pumped output potential from being influenced during the short-circuiting of the second electrodes of the pumping capacitors.
It is favorable if the control signal is periodic. This has the result that every pumping cycle takes place in the way according to the invention.
The first and second switching elements may contain, for example, switching elements such as transistors or diodes for example.
According to a development, the charge pump has a first switching element, via which the control terminal of the short-circuiting element is connected to the second electrode of the first pumping capacitor, and a second switching element, via which the control terminal of the short-circuiting element is connected to the second electrode of the second pumping capacitor. The two switching elements make it possible to adapt the potential at the control terminal of the short-circuiting element to the potential of one of the two second electrodes at desired points in time.
In accordance with an added feature of the invention, a third switching element is connected between the control terminal of the first switching element and the supply potential terminal, and through the third switching element the control terminal of the first switching element is coupled to the supply potential terminal. A fourth switching element is connected between the control terminal of the first switching element and the second electrode of the second capacitor, and through the fourth switching element the control terminal of the first switching element is coupled to the second electrode of the second pumping capacitor. A fifth switching element is connected between the control terminal of the second switching element and the second electrode of the first pumping capacitor, and through the fifth switching element the control terminal of the second switching element is coupled to the second electrode of the first pumping capacitor. A sixth switching element is connected between the control terminal of the second switching element and the supply potential terminal, and through the sixth switching element the control terminal of the second switching element is coupled to the supply potential terminal.
In accordance with an additional feature of the invention, the third switching element has a control terminal connected to the control terminal of the controllable short-circuiting element. The sixth switching element has a control terminal connected to the control terminal of the controllable short-circuiting element. The fourth switching element has a control terminal connected to the control terminal of the second switching element. The fifth switching element has a control terminal connected to the control terminal of the first switching element.
In accordance with a concomitant feature of the invention, a third pumping capacitor has a first electrode connected to the control terminal of the first switching element and a second electrode for receiving a first pumping signal. A fourth pumping capacitor has a first electrode connected to the control terminal of the controllable short-circuiting element and a second electrode for receiving a second pumping signal. A fifth capacitor has a first electrode connected to the control terminal of the second switching element and a second electrode receiving a third pumping signal.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a charge pump, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.